The present invention relates to a method and apparatus for detecting airborne radioactive contamination, and is particularly concerned with a continuous air monitor (CAM) having the ability to accurately measure transuranic and similar particulate concentrations in the presence of the radioactive decay of naturally occurring inert gasses, such as radon and thoron, and their daughter products (progeny).
The monitoring of airborne radioactive contamination is a critical aspect of the control and/or processing of many materials. One of the areas of technology where monitoring the presence of airborne radioactive contamination is of great concern involves the physical handling and disposing of materials classified as radioactive waste. Other areas of concern relate to weapons manufacturing and the process of nuclear fission, as the byproducts of these respective activities can be quite hazardous. The safety of workers in these areas and of the public in general is dependent on the ability to quickly detect even trace amounts of hazardous radioactive material released into the environment. When safe levels are exceeded, it is desirable to automatically trigger an alarm so as to warn personnel in the vicinity of the radioactive emission, for in some cases automatic or manual emergency procedures must be implemented to combat the emission.
An obstacle to the process of accurately measuring these hazardous materials is the pervading presence of radon and thoron gas and their short-lived daughter products. Both radon and thoron gasses, as well as many of their short-lived daughters, emit alpha energy, whereas other of the daughters emit beta energy. In air monitoring systems which use particulate filters, the daughter products of radon and thoron are collected on the filter paper in addition to other alpha-emitting particles (e.g., plutonium, americium and neptunium) and contribute to the total alpha energy measured on the filter. However, radon and thoron gasses themselves do not collect on the filter paper; instead, they pass through the filter with other gas products. The radon and thoron decay chains are shown in Table I and Table II, respectively.
TABLE I ______________________________________ Radon Decay Chain Common Common Isotope Isotope Name Symbol Name Symbol ______________________________________ radium Ra radium-226 .sup.226 Ra radon Rn radon-222 .sup.222 Rn radium A RaA polonium-218 .sup.218 Po radium B RaB lead-214 .sup.214 Pb radium C RaC bismuth-214 .sup.214 Bi radium C' RaC' polonium-214 .sup.214 Po radium D RaD lead-210 .sup.210 Pb radium E RaE bismuth-210 .sup.210 Bi radium F RaF polonium-210 .sup.210 Po radium G RaG lead-206 .sup.206 Pb ______________________________________
TABLE II ______________________________________ Thoron Decay Chain Common Common Isotope Isotope Name Symbol Name Symbol ______________________________________ radium Ra radium-224 .sup.224 Ra thoron Rn radon-220 .sup.220 Rn thoron A ThA polonium-216 .sup.216 Po thoron B ThB lead-212 .sup.212 Pb thoron C ThC bismuth-212 .sup.212 Bi thoron C' ThC' polonium-212 .sup.212 Po thoron D ThD lead-208 .sup.208 Pb ______________________________________
The III presents the alpha energies and half-lives of materials of typical interest in continuous air monitoring. As a single isotope can exhibit a multitude of decay patterns, the relative percentage of each particular mode of isotope decay is listed under the "comments" column, where appropriate. Such isotopes are commonly referred to as being "multi-modal". Due to rounding errors and the existence of "non-alpha particle-emitting modes of decay" for particular isotopes, the listed percentage for the modes of isotope decay may not total 100%.
TABLE III ______________________________________ ISOTOPE DATA Isotope Half-life Alpha Energy Comments ______________________________________ .sup.222 Rn 3.82 days 5.490 MeV Radon .sup.218 Po 3.05 minutes 6.002 MeV RaA .sup.214 Pb 26.8 minutes no alpha RaB .sup.214 Bi 19.7 minutes no alpha RaC .sup.214 Po 164 .mu.sec 7.687 MeV RaC' .sup.220 Rn 55 sec 6.287 MeV Thoron .sup.216 Po 150 msec 6.777 MeV ThA .sup.212 Bi 60.6 min 6.051 MeV ThC .sup.212 Po 300 nsec 8.785 MeV ThC' .sup.239 Pu 24,400 years 5.105 MeV 12% .sup.239 Pu .sup.239 Pu 24,400 years 5.143 MeV 15% .sup.239 Pu .sup.239 Pu 24,400 years 5.156 MeV 73% .sup.239 Pu .sup.238 Pu 86 years 5.456 MeV 28% .sup.238 Pu .sup.238 Pu 86 years 5.499 MeV 72% .sup.238 Pu .sup.241 Am 458 years 5.443 MeV 13% .sup.241 Am .sup.241 Am 458 years 5.486 MeV 86% .sup.241 Am .sup.243 Am 7950 years 5.276 MeV 88% .sup.243 Am .sup.243 Am 7950 years 5.234 MeV 11% .sup.243 Am .sup.244 Cm 17.6 years 5.763 MeV 23% .sup.244 Cm .sup.244 Cm 17.6 years 5.806 MeV 77% .sup.244 Cm .sup.245 Cm 9300 years 5.362 MeV 80% .sup.245 Cm .sup.245 Cm 9300 years 5.306 MeV 7% .sup.245 Cm .sup.237 Np 2,140,000 yrs 4.765 MeV 17% .sup.237 Np .sup.237 Np 2,140,000 yrs 4.770 MeV 19% .sup.237 Np .sup.237 Np 2,140,000 yrs 4.787 MeV 51% .sup.237 Np ______________________________________
With reference to Tables I and III, RaA, RaB, RaC and RaC' have half-lives of less than a half hour. In contrast, RaD has a half-life of 22 years. RaA, RaB, RaC and RaC' are therefore known as the short-lived daughters of radon. Because their half-lives are so much longer than that of the short-lived daughters of radon, the activity of RaD, RaE and RaF can be ignored in typical measurement situations. The final element in the decay chain is lead-206, which is a stable element.
Table III illustrates that the half-lives of the transuranic elements (plutonium, americium and neptunium) are orders of magnitude larger than those of the radon and thoron daughter products. The longest lived of the radon and thoron daughter products, Bismuth-212 (ThC), has a half-life of about one hour (61 minutes). Existing radioactive monitoring applications are constructed in such a manner that the short half-life of the radon and thoron daughters and the long half-lives of the transuranics and other radioactive elements of interest work to the disadvantage of measurement sensitivity and accuracy. The decrease in the sensitivity and accuracy of the radioactivity measurement is due to the rapid build-up of thoron and radon daughter products on the filter area being measured and the high radioactive energy count rate of small quantities of these daughters products compared to other elements such as the transuranic elements present on the filter area which have slower build-up times.
Because of their extremely short half-lives, there is very little .sup.214 Po (RaC'), .sup.216 Po (ThA) or .sup.212 Po (ThC') in ambient air. These elements decay into their immediate daughters almost as quickly as they are formed. Therefore, since these respective isotopes are not in existence long enough to interfere with the radioactive measurements being taken, there is no need to remove them from the air stream to be measured. Furthermore, these isotopes are characterized by relatively high energy alpha emissions in excess of about 6.78 MeV. These energies are well above those of the transuranic elements, which have a peak alpha energy level of about 5.81 MeV. By discriminating the alpha energies, it is possible to recognize the source of the alpha particles. Proper design practices and minimization of filter packing can minimize the effect of counts from these isotopes. The .sup.214 Po can, in fact, provide positive benefits as a marker pulse to indicate energy location and distribution.
The two radon/thoron daughters that cause the most trouble with regard to obtaining an accurate measurement of the radioactive energy present in an air stream, are .sup.212 Bi (ThC) and .sup.218 Po (RaA). These two isotopes have the lowest alpha energies of the radon and thoron daughters and therefore influence the measurement of the transuranic elements the most. In most parts of the country, radon daughters are significantly more plentiful than thoron daughters. Fortunately, because of its short half-life, the contribution of RaA to the total alpha counts generated by radon daughters is only about 10% of the total. Although alpha counts attributable to RaA constitute a relatively minor portion of the total radon daughter alpha energy collected on a filter paper, these counts can still be much more numerous than those that are attributable to the transuranic counts. There have been several techniques that have been developed in an attempt to separate the radon and thoron daughters from the sample air stream prior to its passage to the filter. However, most of these techniques involve somewhat complicated air flow and equipment configuration.
In relatively clean particulate environments, an appreciable portion of the RaA is not attached to dust particles. Typical fractions of unattached RaA are in the range of from about 50% to about 90%. However, the fraction of RaA that is unattached can vary considerably in accordance with the amount of particulate material in the sample of air that is being tested. In accordance with the design of the air particulate collection system, some of the radon and thoron progeny can be removed. Plates and screens (both charged and uncharged) have previously been used to remove a portion of the radon daughter products before they are able to deposit on the filter paper. None of these known removal methods is 100% effective, however, and an appreciable percentage of the radon and thoron daughter products pass through to the filter paper. For example, employees of various government regulatory agencies have for some time used simple wire mesh (on the order of 60.times.60 per inch) to remove major portions of the unattached RaA.
The current detection requirement in many localities for work-place and stack emissions for plutonium (a common transuranic element of interest) provides for a DAC (derived air concentration) level of 2.times.10.sup.-12 .mu.Ci/ml (2.times.10.sup.-3 pCi/l). The DAC level for radon and its daughters is set at 3.times.10.sup.-8 .mu.Ci/ml (30 pCi/l). Actual radon levels in above-ground facilities are seldom above 4 pCi/l and are usually at or below 2 pCi/l. Even at 2 pCi/l however, the radon alpha activity is three orders of magnitude higher than that for plutonium.
As has been noted above, efforts are usually undertaken to discriminate between the particle energies emanating from the radon and thoron daughters and the particle energies emanating from the primary particles of interest (e.g., transuranics) when attempting to monitor the levels of airborne radioactive contaminants. In some cases, the energies that are to be monitored are divided into two spectrums. However, this approach has been found to be too inaccurate for the reasons specified above. Alternative monitoring techniques using multichannel analysis are frequently used in present particle energy discrimination applications. The typical multichannel analyzer divides energies to be monitored into 256 individual energy ranges. As noted above and in Table III, the highest alpha energy levels of the emissions of the transuranic materials are less than that of the lowest energy alpha particle emanating from the short-lived radon and thoron daughters. However, because of physical geometries, detector imperfections, particle burying into the filter paper, and particle covering by dust on the filter paper, the precise and repeatable measurement of particular alpha energies can be compromised, the aforementioned disparity in energy levels for the emissions of the transuranic and radon and thoron decay products notwithstanding. The energies of some of the lower radon and thoron daughter product alpha energies may be measured within the energy range of the higher energy transuranic alpha particles, thereby resulting in the generation of false alarms which can have adverse and costly consequences. These shortcomings have led to the development of the continuous air monitoring system of the subject application.